GaN-based semiconductors are generally applied to optical devices such as a blue/green LED and high-speed switching and high-power electronic devices such as a Metal Semiconductor Field Effect Transistor (MESFET) and a High Electron Mobility Transistor (HEMT). In particular, blue/green LEDs have been mass produced lately, and their worldwide demand is rising sharply.
A GaN-based semiconductor light emitting device is typically grown on a substrate of sapphire or SiC. Then, a polycrystalline layer of AlyGa1−yN is grown as a buffer layer on the sapphire or SiC substrate at a low growth temperature. At a higher temperature, an undoped GaN layer and a Si-doped n-type GaN layer or a mixed structure thereof are grown on the buffer layer to provide the n-type GaN layer as a first electrode contact layer. Then, a Mg-doped p-type layer is formed as a second electrode contact layer thereon to produce a nitride based 3-5 group compound semiconductor light emitting device. In addition, an active layer (of a multiple quantum well structure) is interposed between the n-type first electrode contact layer and the p-type second electrode contact layer.
In the nitride based 3-5 group compound semiconductor light emitting device of this structure, crystal defects found in the interface between the substrate and the buffer layer have a very high value of about 108/cm3. As a result, this degrades electrical characteristics of the nitride based 3-5 group compound semiconductor light emitting device, and more particularly, increases leakage current under reverse bias conditions, thereby causing a fatal effect to the reliability of the light emitting device.
In addition, the crystal defects created in the interface between the substrate and the buffer layer degrades the crystallinity of the active layer, and therefore disadvantageously lowers the luminous efficiency of the nitride based 3-5 group compound semiconductor light emitting device.
In the meantime, in order to improve the performance and reliability of the GaN-based semiconductor light emitting device, researches have been made for new buffer layers and various fabrication methods of GaN-based semiconductors have been studied.